I went through the whole "just friends" thing with my XW. They love to have their cake and eat it too. I eventually booted her @ss out the door, but I waited far too long. In my opinion, the sooner you boot 'em out the door, the higher the probability that they'll come back. In addition, his girlfriend is a beneficiary of the current situation as well; she gets to have all of the fun without the maintenance and commitment. I'd bet that as soon as his girlfriend realizes that your husband is all hers, then her feelings will change very quickly.
The other interesting dynamic is that once they're gone, you begin to realize that, just maybe, they weren't really the wonderful spouse that you believed them to be. Love has a strange way filtering reality in favor of the loved one.
